Polyvinyl alcohol (PVOH) is a synthetic resin generally prepared by the alcoholysis, usually termed hydrolysis or saponification, of polyvinyl acetate. Fully hydrolyzed PVOH, where virtually all the acetate groups have been converted to alcohol groups, is a strongly hydrogen-bonded, highly crystalline polymer which dissolves only in hot water—greater than about 140° F. (60° C.). If a sufficient number of acetate groups are allowed to remain after the hydrolysis of polyvinyl acetate, the PVOH polymer then being known as partially hydrolyzed, it is more weakly hydrogen-bonded and less crystalline and is soluble in cold water—less than about 50° F. (10° C.). Both fully and partially hydrolyzed PVOH types are commonly referred to as PVOH homopolymers although the partially hydrolyzed type is technically a vinyl alcohol-vinyl acetate copolymer.
The term PVOH copolymer is generally used to describe polymers that are derived by the hydrolysis of a copolymer of a vinyl ester, typically vinyl acetate, and another monomer. PVOH copolymers can be tailored to desired film characteristics by varying the kind and quantity of copolymerized monomers. Examples of copolymerizations are those of vinyl acetate with a carboxylic acid or with an ester of a carboxylic acid. Again, if the hydrolysis of acetate groups in these copolymers is only partial, then the resulting polymer could be described as a PVOH terpolymer—having vinyl acetate, vinyl alcohol, and carboxylate groups—although it is commonly referred to as a copolymer.
It is known in the art that many PVOH copolymers, because of their structure, can be much more rapidly soluble in cold water than the partially hydrolyzed type of PVOH homopolymers. Such copolymers have therefore found considerable utility in the fabrication of packaging films for the unit dose presentation of various liquid and powdered products including agrochemicals, household and industrial cleaning chemicals, laundry detergents, water treatment chemicals and the like.
Examples of such copolymers are those prepared by the hydrolysis (base catalyzed alcoholysis) of copolymers of vinyl acetate and carboxylic acid vinyl monomers, and copolymers of vinyl acetate and esters of carboxylic acid vinyl monomers. If sufficient base is present such that the acid groups (including those resulting from ester hydrolysis) are neutralized to form carboxylate salt groups i.e., ionomer groups, the cold water-solubility of these PVOH copolymers, and hence films fabricated from them, is very rapid. Examples of carboxylate-containing PVOH copolymers derived from monocarboxylic acid vinyl monomers and their esters are those prepared by the hydrolysis of a vinyl acetate-acrylic acid copolymer, a vinyl acetate-crotonic acid copolymer, a vinyl acetate-methyl acrylate copolymer, a vinyl acetate-methacrylic acid copolymer, and a vinyl acetate-methyl methacrylate copolymer, all of which have excellent cold water-solubility. In fact, packaging films based on PVOH copolymers having carboxylate groups are generally considered in the art to be the most rapidly cold water-soluble films.
One of the largest markets for these films is the liquid laundry detergent market where the convenience of the unit dose concept is widely accepted and the rapid solubility of the films is particularly suited to this application.
However, a significant problem exists with the chemical compatibility of virtually all liquid laundry detergent formulations and most of the commercial film used to package these products. The commercial film is based on a carboxylate-containing PVOH copolymer where the carboxylate units, if converted to carboxylic acid groups, are readily able to form stable lactones by cyclizing with adjacent hydroxyl groups. Specifically, the chemical incompatibility derives from the acid-base equilibria that exist in the liquid laundry detergent formulations and are usually in the form of amine-fatty acid equilibria and/or amine-anionic surfactant acid equilibria. Even if the detergent formulation is at an alkaline pH by virtue of the presence of a molar excess of amine, exchangeable hydrogen ions are still available to react with the carboxylate groups of the PVOH copolymer. When this happens, carboxylic acid groups form and they in turn will readily react with adjacent hydroxyl groups to form intramolecular lactones if the lactones have stable five-membered (gamma, γ) ring structures. Other liquid products too numerous to mention may present similar chemical incompatibilities and are, therefore, addressed by the present invention.
The solubility of the polymer and hence the film is markedly affected by this reaction to form lactones; complete insolubility can occur in some cases resulting in polymer residues being attached to items of clothing at the end of a wash cycle. All the above-mentioned carboxylate-containing copolymers derived from monocarboxylic acid vinyl monomers and their esters are subject to this reversion to stable γ-lactones in the presence of hydrogen ions. There is therefore a need to provide cold water-soluble films, preferably including those based on PVOH copolymers having carboxylate functionality (because of their fast solubility) where the functional groups of the copolymers which facilitate cold water solubility are substantially unaffected by hydrogen ions or, if chemically modified by reaction with hydrogen ions, the modified functional groups do not significantly affect the water solubility of the films.